1. Field of the Invention
This invention relates to semiconductor devices and, more particularly, to semiconductor devices responsive to electromagnetic radiation.
2. Art Background
A significant measure of the usefulness of a system employing opto-electronic devices (photodetectors) or photovoltaic devices (solar cells) is the efficiency of the system. Indeed, even small increases of the efficiency in optical communications systems (opto-electronic device system) or in solar energy systems (photovoltaic device systems) are extremely important. The primary method for increasing efficiency in such systems is by decreasing scattering loss in the optical fibers of the opto-electronic system or by increasing the solar conversion efficiency in the photovoltaic system. Various schemes are utilized to achieve increased efficiency through these methods. For example, in the case of opto-electronic systems, scattering in the glass fiber is reduced by employing longer wavelength radiation. In this regard, optical communication devices that detect light at 1.05 and 1.3 .mu.m have been fabricated using semiconductor materials with appropriate bandgaps. Similarly, the efficiency of solar energy systems has been improved by an appropriate choice of constituent semiconductor materials having a desirable bandgap--the bandgap strongly influences the developed current and voltage, and thus, the solar conversion efficiency.
Because the bandgap of ternary semiconductor alloy materials is controlled by adjusting their composition, these complex semiconductor materials are used in various opto-electronic and photovoltaic devices to achieve the goal of increased efficiency. (See C. J. Nuese, Journal of Electronic Materials, 6, 253 (1977) and R. L. Moon, L. W. James, H. A. VanderPlas, T. O. Yep, G. A. Antypas, and Y. Chai, Conference Records 13th IEEE Photovoltaic Specialist Conference, Washington, D. C. 1978, respectively, for exemplary opto-electronic and photovoltaic devices using ternary semiconductor alloy material.) Generally, the best composition of a ternary alloy material for a given application can be determined without extensive experimentation. Once an efficiency for a device using a ternary semiconductor material of one composition having a given energy gap is measured, reliable theoretical prediction of the efficiencies obtained with the same ternary having a different composition and concomitant energy gap usually is made. (See J. J. Loferski, Journal of Applied Physics, 27, 777 (1956).) In this manner the composition yielding the best efficiency is generally determined by fabricating only one or two devices with different compositions. Similarly, from the bandgap and photo response measured for a ternary used in an opto-electronic device, the photo response for other composition of the same ternary system is reliably predicted. Despite this situation, the number of photovoltaic device having efficiencies of at least 15 percent is quite limited and there exists only a very limited number of opto-electronic devices which respond to wavelengths in the near to intermediate infrared, i.e., wavelengths beyond 1.65 .mu.m.